This invention relates to horizontal deflection circuits in television receivers.
Video signals, representative of an image to be displayed, are processed by the television receiver for display on the screen of a cathode ray tube. The composite video signal includes signals representative of an image to be displayed and includes timing information in the form of sync pulses superimposed upon the blanking portions of the video signals.
The video information is displayed on the CRT screen through modulation of one or more electron beams deflected vertically and horizontally to form a raster. Horizontal deflection of the electron beam is accomplished by generating a linear sawtooth horizontal deflection current in the horizontal deflection winding. The midpoint of the horizontal scan line should coincide with the horizontal center of the screen. This point usually occurs when the deflection current is zero.
Various raster distortions may result under conditions of heavy video loading of the high voltage circuits which provide accelerating potentials for the electron beams. One distortion, known as blooming, results in symmetrical variations in size of the raster caused by a decrease in high voltage during instants of heavy video loading. An increase in raster width caused by blooming may be offset by decreasing the B+ voltage under heavy loading. The B+ voltage is used as a driving voltage for the horizontal deflection winding to provide a sawtooth scanning current. A decrease in B+ voltage results in a decrease in peak-to-peak scanning current providing for blooming compensation.
Another raster distortion may arise if there are any shifts in the average horizontal beam current under heavy video loading. As taught by U.S. Pat. No. 3,959,689, granted to J. Ikoma, video loading of the high voltage circuits results in greater energy being coupled from the retrace pulses of the horizontal deflection winding to the high voltage circuit by the horizontal output transformer. This causes a shifting of the average horizontal deflection current and a physical distortion of the raster as the horizontal lines of the raster shift relative to the cathode ray tube screen. A voltage analogous to that of the average horizontal deflection current is obtained by sampling the electron beam current. The voltage is coupled to the horizontal oscillator in order to change the oscillator frequency in a manner which compensates for the distortion.
The origin of additional raster distortion is taught in U.S. Pat. No. 3,426,244, granted to G. Strachanow. Heavy video loading may cause a widening of the retrace pulses coupled as comparison pulses to the phase discriminator diodes of a phase detector. The phase detector undesirably responds to the increased width of the retrace pulses by generating an error voltage for changing the horizontal oscillator frequency, causing a shift of horizontal lines and a distortion of the picture display. A compensating voltage obtained by sampling the current through the horizontal output transistor is developed across a resistor and is coupled to the junction of the phase discriminator diodes, which cancels the error voltage and corrects for the distortion.
Still another raster distortion is caused by variations in storage time delay in the cutoff of the horizontal output transistor after its base-emitter junction is reversed biased, resulting in variations in the initiation of the retrace interval. Because of these variations, the deflection current is no longer synchronized with the sync pulses, and the video information is no longer displayed in the proper positions in each line of the raster, causing a distorted display.
One method of compensating for this distortion, as disclosed in U.S. Pat. No. 3,891,800, granted to P. J. H. Janssen, et al., is to couple together in a second phase comparator the retrace pulses and the output signal of an oscillator synchronized with the sync pulses. The output of the second phase comparator serves as an error voltage for adjusting the frequency of the horizontal oscillator to compensate for the distorted display. Two phase comparators and two oscillators are required for such a correction arrangement.